Peening is a well-known process to improve the material properties of a metal. The peening impact, usually created by mechanical means such as a hammer blow or by a blast of shot (e.g., shot peening), plastically deforms the metal surface to produce residual compressive stresses at or below the surface and tensile stresses in the interior. The compressive stresses in the metal surface improves the metal's resistance to metal fatigue and crack growth.
A laser pulse may be used in place of a hammer blow or blast of shot to provide the peening impact on the metal surface. In a typical layer shot peening system, an opaque coating such as black tape or paint is applied to metal surface to form an ablative coating. A translucent layer, usually in the form of a flow of water, on top of the ablative coating acts as a tamp. Short laser pulses focused on the target surface explode the ablative coating, and the translucent layer directs the resulting shock wave into the surface of the target material. The laser beam may then be repositioned and the process repeated to create an array of slight indents of compression and depth in the surface of the target material. The repositioning of the laser pulse is often computer or robotically controlled to precisely direct the laser pulses at specific locations on the surface of the target material. Laser shock peening typically produces a layer of residual compressive stress near the target surface that is four times deeper than that attainable from conventional shot peening treatments.
A laser shock peening system often includes sensors and circuitry to measure the amount and location of energy deposited in the target material. For example, a piezo-electric transducer attached to the target material may be used to sense the shock wave produced by the laser pulse and produce an electric current that is proportional to the shock wave. A known disadvantage of piezo-electric transducers, however, is that the shock wave produced by the laser pulse often damages the piezo-electric transducer. As a result, multiple piezo-electric transducers are required for each laser shock peening cycle, and the ability to continuously monitor the effectiveness of the laser shock peening is limited.
Another disadvantage associated with using piezo-electric transducers to monitor the effectiveness of laser shock peening is a lack of sensitivity to slight malfunctions in the laser shock peening process that reduce the effectiveness of the process. For example, studies have shown that piezo-electric transducers lack the sensitivity to reliably identify deficiencies in the translucent layer, ablative layer, or energy level of the laser pulse. Any one of these deficiencies in the system may reduce the amount of energy deposited on the surface of the target material, with a corresponding decrease in the amount and depth of compressive stress produced in the target material. As a result, piezo-electric transducers are unable to reliably identify malfunctions in the laser shock peening process.